An aircraft is kept airborne by the aerodynamic lift of its wings.
An aircraft wing comprises a main wing, and in many cases also lift-assisting devices fixed to said wing. A lift-assisting device is a device on a wing of an aircraft, which device positively changes the lift coefficient at least in a range of the flight spectrum.
Lift assisting devices are in particular used during landing and during takeoff of an aircraft. The aim is, as a result of the increased lift, to reduce the take-off speed or landing speed and thus reduce the distance required for take-off or landing.
Lift-assisting devices may be affixed to the leading edge or the trailing edge of an aircraft wing. The so-called Fowler flap is an important example of a lift-assisting device affixed to the trailing edge of a wing. A Fowler flap is a control surface which is moved to the rear below the trailing edge of the wing and is set at an angle. In this way an air gap may be formed between the top and the bottom of the wing, as a result of which the airfoil curvature is increased. At the same time the wing surface is also increased.
FIG. 1 shows a retracted state 100 and an extended state 110 of a Fowler flap 102 affixed to the trailing edge of a main wing 101. In the retracted state 100 the Fowler flap 102 abuts the main wing 101. In order to move the Fowler flap 102 from the retracted state 100 to an extended state 110, the Fowler flap 102 is first moved to the rear and then folded downward. In this way an air gap 111 is created between the main wing 101 and the extended Fowler flap 102. As shown in FIG. 1, the Fowler flap 102 is attached to the trailing edge 103 of the main wing 101.
A Fowler flap which may be extended to form an air gap jointly with a main wing is known. See Rudolph, P “High-Lift Systems on Commercial Subsonic Airliners”, NASA Contractor Report 4746, section 1.1.2. To achieve good flow characteristics with a Fowler flap it is important that the size of the air gap created when the flap is extended be well defined, and that a divergent air gap over the entire region of the Fowler flap be prevented.
This requirement may be met by various kinematic solutions. According to the state of the art, the so-called track and rear-link solution (for example implemented in the Airbus A340) or the 4-bar linkage solution (for example implemented in the Boeing 777) is used. See Rudolph, P “High-Lift Systems on Commercial Subsonic Airliners”, NASA Contractor Report 4746, section 1.2.2.
Pivot point kinematics (pivot point or dropped hinge), according to which the flap is extended along a circular path is used in the Boeing C17. An example of the Fowler flap 200 is shown in FIG. 2. The Fowler flap 200 is brought along a circular direction of extension 201, starting from a retracted state 100 to an extended state 110.
Track and rear link kinematics shows good performance in relation to aerodynamic characteristics. Pivot point kinematics provide advantages in relation to the complexity of the system, which also results in reduced weight.
However, both the track and linkage technology and the pivot point technology are associated with disadvantages. Due to its limitation to a circular extension path, pivot kinematics only allow the setting of a desired target state when the flap is in position. The width and the form of the gap for the intermediate states during extension result automatically and cannot themselves be set. As a rule, the settable target setting is set such that in the fully extended state 110 it produces a predefinable result. In relation to intermediary extension states the width of the air gap often takes on a value that is below the optimum value, as a result of which the quality of the functionality of the flap may be reduced, and in particular the flow characteristics of the flap may be impeded (risk of confluent boundary layer flow). Due to the circular movement the shape of the gap in an intermediate state is partly divergent. Due to local deceleration of the flow speed this leads to separation of the boundary layer flow at the flap, with subsequent deterioration of lift performance, which in addition can also lead to the occurrence of vibration and noise. In a worst-case scenario this effect can lead to a range of intermediary flap positions not being useable at all.
Furthermore, in an advanced (high) degree of extension of the flap, irrespective of the kinematics used, boundary layer separation at the flap element can occur. This effect limits the efficiency of the flap, defines the maximum usable flap angle and causes vibration of the flap elements at high angles of extension.
There is a longstanding and unresolved need to provide an aircraft wing in which the lift performance of the wing is improved, and undesirable vibration and noise are prevented.